Miniaturization of optical, electro-optical, and optoelectronic components and modules is reaching a stage where complex optical, electro-optical, and opto-electronic functionalities may be realized on a single semiconductor chip termed “photonic integrated circuit”. A photonic integrated circuit may include optical waveguides and other micro-optical structures.
Photonic integrated circuits may be used for separation, modulation, demodulation, and detection of optical signals, making them attractive for optical communications systems. Furthermore, photonic integrated circuits may be compatible with electronic circuitry, which enables such functions as transmission, reception, and modulation of light on a single chip.
Despite the progress of optical integration of multiple functionalities of photonic integrated circuits, the task of coupling light between different waveguides of a same or a different photonic integrated circuit, and between a photonic integrated circuit and an optical fiber remains challenging. Optical modes guided by planar waveguides of different size and/or different refractive index contrast may differ considerably in size and shape. An optical waveguide mode is usually much smaller in size than an optical mode guided by a singlemode optical fiber or fibers, which are used to optically couple a photonic integrated circuit to an outside environment. A semiconductor-based optical mode converter may be used to provide conversion between optical modes of different sizes, shapes, and different vertical positions relative to the semiconductor substrate.
One prior-art solution of a problem of an optical mode conversion and vertical displacement includes using vertical couplers to couple light from a lower optical waveguide to a differently sized upper optical waveguide, or vice versa. Another solution is to use waveguide tapers having physical thickness varying in vertical direction, and/or a width varying in a horizontal direction. These techniques are rather costly and may be difficult to implement in production environment, especially for vertical direction.
Waveguide tapers are perhaps most frequently used for conversion between different optical mode sizes of planar waveguides. Waveguide tapers may also be used for coupling light between a waveguide and an external optical fiber. However, waveguide tapers typically have to be made long enough to ensure an adiabatic mode transformation to avoid considerable optical losses. Long waveguide tapers tend to occupy a considerable area on a photonic chip, especially if an array of such tapers is required to optically couple an array of optical fibers to a photonic chip.
Therefore, the prior art appears lacking a manufacturable and reproducible semiconductor optical waveguide device capable of optical mode size conversion and/or vertical displacement of optical modes.